Method of making manganese



April 16, 1946. T. AQMITCHELL 2,393,589

METHOD OF MAKING MNGANESEl FiledvJan.. 11, 1959 Lin-:11 Qf Ramsau Malfa-abs Patented Apr. 16, 1945 METHOD F IVIAKIN G MANGANESE Thomas A. Mitchell, Torrance, Calif., assignor,

by mesne assignments, to Molybdenum Corporation of America, New York, N. Y., acorporation of Delaware Application January 11, 1939, serial No. 250,395

(ci. zot-s4) 4 Claims.

This invention relates to a method of making metallic manganese and which, in particular, is'

an electrothermic method of producing manganese from its oxides.

Various futile attempts have been made heretofore to form manganese metal commercially by electrolysis of a fused bath containing a manganese compound. For example, the electrolysis of fused manganese fluoride did not produce vthe metal. (Mellors Inorganic and Theoretical Chemistry, vol. XII, 1932, page 165).. Likewise,

electrolysis of a manganese lore containing 69.36% Mn3O4, 2.2% FezOa and 9.31% SiOz with 25% of silica at 1250 C. gave a deposit of iron containing only 0,8% manganese. (Zeitschrift fur Elektrochemie volume 34, 1928, pp. 199-204).

. The electrolysis of a fused bath containing calcium manganite with a carboncathode gave a product containing carbon and other impurities. (Trans. American Electrochemical Society, vol. 19, 1911, page 171). The attempt to make ferromanganese by electrolysis between carbon electrodes of a fused bath of calcium fluoride containing ferro-mangane'se and with the aid of carbon as a reducing agent gave a product highly contaminatedwith carbon. For example, with a raw charge of 6 parts MnaOi, 0.6 FesOr, 3 CaO and 6 SiOz, one reported product had the composition of 83.5 Mn, 8.74 Fe, 0.32 Si and 7.32 C. (Zeitschrift fur Elektrochemie vol. 8, No. 20, 1902, page 302; Wrights Electrical Furnaces and their Industrial Application, 1905, pages 209 to 211).

It has also been suggested that metallic manganese could be obtained by an alumino-thermic reaction in which powdered aluminum metal is caused to react withy manganese oxide to reduce the latter to the metallic form. It is difllcult to get an aluminum free product by this method;

and if a large excess of the manganese oxide is used to avoid contamination with aluminum, there is a resultant loss of material which is not easily recovered from the slag. This process is used commercially, but its eld is very limited due to the high'cost of production. Also, the direct reduction of manganese oxide by carbon in a high temperature furnace has serious disadvantages. For example, the commercial production of ferro-manganese requires a temperature of 1350 C. or higher, and at this high temperature,

there is a strong ainnity between the manganese ft and carbon so that the final metallic product may contain from 6 to 8% o f carbon. The manganese carbide thus formed is unstable in the presence of moist air, and this requires treatment with decarburizng agents for the removal of the carbon, thus entailing an additional operating expense and loss of vmanganese as well as the possibility of contaminating the final product with reagent impurities. There is also the loss of manganese by Volatilization as well as the cost of maintaining the desired temperature conditions. Various other disadvantages are inherent in such prior art proposals.

In the production of steels containing manganese, it is often desirable that the manganese be low in carbon so as to obtain a steel of required characteristics. On the other hand, the presence of iron as an impurity may not be detrimental. For example, there is a commercial demand for manganese metal containing not over 1% of silicon, 2% of iron and 0.5% of carbon. There is also a need for substantially pure manganese for various other industrial uses.

The primary object of this invention is to provvide` a method which will serveior the manufacture of either substantially pure manganese or a commercial product which has a. suillciently high purity to be useful in various industries.

A further object is to make metallic manganese which has a low carbon content, or is substantially free from carbon and carbides as well as detrimental amounts of various other impurities.

A further object is to make low carbon manganese metal by a single step process which is economical, eiiicient and capable of large scale production of a metal' of required purity. y

In accordance with my invention, I propose to make metallic manganese by the electrolysis of a fused bath of alkaline metal Vfluoride containing dissolved therein a suitable compound of a reagent metal, such as aluminum or magnesium, which is capable of being normally deposited electrolytically from such a bath, and which reacts under the conditions of the process with a suitable manganese compound supplied for the purpose to cause manganese metal to be formed and collected in the cell.

Another object of the invention is, therefore,

ganese oxide in solution will react chemically therewith to form metallic -manganese, while the the metallic form. Aluminum and magnesium,` as

well as the alkali metals and alkaline earth metals, which are herein termed alkaline metals. are electropositive to manganese and the aluminum and magnesium are capable of reducing manganese oxide under the conditions of the process. so that manganese alone will be the final product and the reagent metals will remain as compounds dissolved in the fused bath. Hence, the process is so carried on as to maintain an excess of manganese oxide dissolved in the fused bath, while a sufficient amount of suitable aluminum or magnesium compound is dissolved therein to provide the reagent metal for the reduction of the manganese oxide.

The electrolytic process is carried on by means of a suitable anode and preferably with a noncarbon cathode, the latter being a, metal which will not react detrimentally with the manganese collected at the cathode. The fused bath may be made of any suitable solvent for the reagent metal oxide and the manganese oxide, and particularly a substance-containing both aluminum and an alkaline metal combined with uorine, such as cryolite (NaaAlF) or a mixture of aluminum iluoride and alkaline metal iluoride, which will have a suitable melting point and satisfactory fluidity. chemical stability and elec trical conductivity at the operating temperature of the bath, and which will otherwise serve as a solvent for these materials and permit the desired reaction. The source of manganese may be l any one of its oxides, but for the sake of eiliciency it is desirable to utilize the lower oxide MnO. Aluminum or magnesium metal will react material which forms the cathode. Suitable insulation 20 separates this cathode from the metal structure forming the cell chamber. Mounted above the cathode, is an anode 22 made of suitable material, such as carbon. This anode may be suitably suspended by means of a clamping ring 24 and other adjustment mechanism arranged for raising and lowering the same as required. As the anode is consumed by oxidation, it maybe suitably lowered and various expedients may be adopted for replenishing the anode as desired. One may use either a multiple anode made of small cylindrical graphite rods or one may use a single large graphite body. as shown in the drawing. This anode is preferably made up in sections screwed together at the top and lowered into the bath as needed. Power may be applied to the cell by means of a clamping ring 26 to which is attached an electrical conductor for delivering a direct current thereto. An elect'rical connection may be made to the cathode 20 by means of a roller 28 suitably mounted to ride against the upturned peripheral flange of the cathode which projects outwardly beyond the cell walL The fused bath may be stirred by various types of mechanism and arrangement of parts, such as a rotating anode; but I prefer that the cathode be rotated, with its attendant advantages. To this end, I may support the metal base I2 of the cell on a lower platform 30 provided with a set of rollers 3l suitably mounted thereon and bearing against a track 32 on the underside of the plate I2. A pivot pin 33 is axially mounted in a boss on the underside of the plate I2 and supported in a boss 34 forming the center portion of the platfonn 30, soy that the cell bottom l2 with each one of these oxides in the metal fluoride bath, but less reagent metal i's required for reaction with the lower oxide than with the others to produce a given amount of manganese, and the use of this lower oxide therefore cuts down the cost of the process.

The process may be carried on in various types of apparatus, but I prefer to employ that shownv in the drawing, which is a diagrammatic vertical section of an electrolytic cell so arranged that the cathode may be rotated as well as moved vdownwardly away from the anode asan ingot Aoi! f `rn ianganese metal builds up in the bottom -l cell.

Ofitlfle ""'Ilhe construction illustrated comprises aset of steel ringsv I0 of U-shaped or channel vmetal. which may be made in sections forthe sake of replaceability, and these rings aret built up one above the other to form the side wall of the cell. These side wall rings are supported on-a bottom platform comprising a metal base plate I2 on which is mounted a layer of insulating bricks Hof silocel or the like, and above which is a further layer made of suitable insulating material I 6, such as that sold under the trade name of Transita Above'this is a metal plate I8 of iron, copper or other suitable non-carbon will be rotatably guided by the pin. A ring gear 35 on the underside of the plate I2 is driven by means of a. gear wheel 36 rotated by any suitable source of power or by hand mechanism. By this simple arrangement, the entire cell, its fused bath and the cathode are rotated relative to the anode. thus causing the molten bath to move about the single or multiple anode rods and depolarize it by removal of the gas from its surface. Also the swirling action within the electrolyte aids in the chemical reaction of the reagent metal on the manganese oxide.

The electrolytic and chemical reactions result in the building up of a mass of manganese metal on the cathode plate. To prevent short circuiting by contact of this deposit with the anode, I propose4 to move the two electrodes away from each other periodically or continually during the process and thereby maintain proper electrolysis conditions within the cell. The anode may be made adjustable in position for this purpose as well as to feed it downwards as oxidation consumes the rod. I prefer, however, to adjust the anode only to compensate for its beingconsumed and to move the cathode downwardly as the ingot builds up. This is done by moving the entire lowered, so as to maintain a proper depth of electrolyte on top of the manganese, which may be in a solidcondition adjacent to the cathode plate.

The cell including the metal cathode carried thereby may be lowered by means of a screw mechanism indicated diagrammatically by the screw 38, which is suitably mounted so that by means of a motor or hand mechanism the cathode and the manganese ingot thereon may be lowered relative to the position of the anode.

I have found that if manganese oxide is fed as a dry powder in large quantities onto the top of the fused bath dll, there is a danger of side reactions taking place between this oxide and the adjacent carbon anode which causes the reduction of the manganese oxide and the formation of manganese carbide, and this may be found present as a contamination in the manganese ingot built up at the bottom of the cell. It is, therefore, desirable that the manganese oxide be fed slowly to the cell as required by exhaustion of the reagent in the solution. This is preferably accomplished by lining the cell wall with a material containing manganese oxide,.and preferably with a mixture of this manganese oxide with alumina or other reagent metal oxide employed and, if desired, with solid material designed to replenish the solvent bath, as that may become necessary Hence, the U-shaped rings I0, which are filled with a suitable insulating material 42, such as magnesia or asbestos, are provided with an innerlining 44 of these desiredreagents and bath ingredients. For this purpose, the U-shaped channels of the rings I maybe closed with plates A,Mi which thus form the inner wall of the cell; and this wall is arranged -to carry a lining of the solid reagent material suitably cemented together and compressed in position and, if desired, baked thereon. This lining may form the soie source of the `reagents and/or bath material, or it may supplement the material fed into the top of the cell.

The circular rings I0 may be lined withthe reagent and bath material either before assembly to form the cell. or after they have been put in,

position. In either case, one may place a temporary cylindrical steel shell or mold inside of the rings I0 and spaced thereon, and then the lining material mixed with a suitable binder is rmly tamped into the annular space between the ring and" the cell wall 46. The sides of the channel members I0 may project inwardly beyond the wall 46, as illustrated, and ,that wall may be corrugated or provided-with ribs or other features which will aid in holding the lining firmly in place. The steel shell is thereafter removed and the lining is preferably baked in position, and this may be done by the heat derived from the cell. This lining material may comprise bath material and reagent metal oxides derived from a previous furnace run mixed with new material as required. Since this lining is dissolved away and thus enlarges the inner cell chamber, it is the balance as a protective covering for the plate 46 and U-shaped channel rings I0.

The operation of the cell may be started by first putting one or two ringsl in place on the cell bottom and then moving the cathode and cell upwardly by means of the screw 38. or by lowering the anode 22, so as to strike an arc between the two electrodes, the voltage being, of course, suitably regulated for that purpose. Aluminum metal may be added initially to form a pool of metal which is ready to react with any manganese oxide sinking to the bottom of the cell. The bath material,- such 'as sodium fluoride and aluminum fluoride, is fed in a dry powdered condition into the cell and is there melted by the heat of the arc. When a sufdciently deep pool of the molten bath of the proper temperature has been attained, the cellis lowered relative to the anode and the voltage is then regulated to cause normal electrolysis and at the same time secure suilicient heat to keep the bath fused Manganese oxide may then be fed gradually into the top of the cell.

An operating voltage of 5 to 8 volts is usually required for the electrolytic action, but the voltage may be held higher than this, or it may be periodically raisedlto 50 or '75 volts or so, and thus furnish enough heat to maintain the bath at the required temperature, which is between 900 and 1100 C. The reaction of aluminum and manganese oxide is exothermic and thus supplies part of the heat. The required bath temperature depends on the bath composition. For example,

- the addition of calcium fluoride will lower its desirable to add further sodium fluoride or other A of the material in solution; and this rate may, therefore, be controlled by feeding a desired amount of manganese oxide to the cell from outside sources, which may be accomplished in any suitable manner so as to avoid deleterious contactwith the carbon anode. In practice, the aim is to operate the cell in such a manner as to consume about two-thirds ofthe lining and leave melting point. Cryolite melts at about 1000 ,C.

and a solution of 16% A1203 therein gives a meltling point of about`935 C. while further additions of alumina raise the melting point so that a saturated solution containing 21% A1203 will melt at about 1000` C. The'bath may contain only a small amount of alumina, such as from 2 to 5%,

with a correspondingly lower melting point.

The passage of a. direct current through the een is believed to' result in aluminum being deposited at the cathode in a molten condition,

bottom of the cell where it builds up an ingot on the cathode and becausefof its high conductivity becomes a part thereof. The manganese may be momentarily in a molten condition (M. P. 1260 C.) if the cell temperature is high enough but it settles out as a solid ingot, pellet or sponge, depending upon the bath temperature and the cell operating conditions. As this metal mass grows, the cell is lowered by means of the screw 38 so as to maintain a proper distance between the anode and the manganese so that the electrolysis may proceed properly. The-lining material dissolves gradually as the manganese is removed from. solution and thereby maintains a solution which is substantially saturated with manganese oxide; and more material for replenishing the bath, as well as the manganese, may be added at the top of the cell, as needed. As vthe metal mass increases, it becomes necessary to build up the Walls of the cell; and this is done by putting more of the rings Ill in position, these being preferably made in sections to permit assembly thereof during the cell operation. The cell operation is, therefore, continuous until all of the rings I have been put into position, within the capacity of the cell structure. When the cell run has been completed the material is allowed to cool and the cell walls then removed. This leaves the mass of manganese exposed and it may be readily separated from the hardened bath material and suitably treated as desired for recovering the manganese. The chilled bath material is then crushed and returned for a succeeding cell run.

The bath is stirred by the evolution of gas and by the rotation of the bath due to electromagnetic induction, and by a mechanical device, if desired. This stirring action tends to keep the solid manganese oxide, which may be added to the bath, from settling to the bottom of the cell. Also, it is found that the molten aluminum, or magnesium if used, is dispersed throughout the bath as a mist of fine droplets.l This formation of mist is desirable, contrary to the standard practice for making metallic aluminum as the nal product, since this highly dispersed material is in position and condition for reacting with the manganese oxide present. Hence, the electrolyte may be thoroughly agitated and the operating conditions may be so controlled as to cause, not only the deposition will not deposit at the cathode to any material extent, and manganese metal which is substantially free from aluminum and magnesium can be 0btalned. 'I he expression an excess of manganese.

oxide, as used in the claims, is to be thus interpreted, and particularly as requiring that there be suflicientmanganese oxide in the bath to insure that the ultimate metal product vbe substan tially wholly manganese.

This freedom of the product from aluminum. is due in part to the fact that the molten aluminum has a very light density as compared with that of the manganese oxide and the manganese metal. That is, the densities o'f the various ingredients in. the fused bath are in the order of fused cryolite 2.095 grams per cc., molten aluminum 2.29, MnzOs 4.5, Mn3O4 4.7, MnOz 5.0, MnO 5.2 and manganese 7.2, while the solution of alumina in the fused cryolite is lighter than the cryolite alone at the same temperature. Hence, this order of the speciflc gravities of the various materials aids in a successful operation of the process. If a mixture of manganese oxides is used, those of lighter density tend to stay in the upper portion of the cell where they react with the aluminum mist, while the heavier particles will concentrate in the lower portions nearer the cathode where they are available for reacting with any aluminum metal depositedV there. This segregation of materials has the advantage thatthe higher concentration of the manganese oxides in the lower part of the cell forces the chemical reactions to completion, so that substantially all of the metallic aluminum is consumed land a manganese of high purity maybe obtained.

As a specific example to further illustrate the process, I employed for the lining'of the cell and the cell feed, a material containing 95% MmOi, 0.6% FezOa, 0.7% SiO: and 3.7% of other constituents'; such as the oxides and sulfates of so` dium. potassium. calcium, magnesium, aluminum,

etc. The fused bath was made up of aluminum fluoride and sodim fluoride; although it will be understood that the fluoride of any .other metal which is electropositive to aluminum may be used, such as'the fluoride of potassium ,or calcium or mixtures of these. Cryolite is preferably used commercially. The average composition of the bath during this run was found to be 29.5% Mn. 19.5% Na, 12.5% Al, 27.5% F, 0.7% Fe and 10.3% of other constituents including small amounts of SiOz, CaO and MgO but mainly oxygen combined with aluminum and manganese. The manganese metal derived from this furnace run, after concentration and remelting, had an analysis of 96% Mn, 1.6 Fe, 0.3 Al, 1.3 Si and 0.8 C. The bath composition as given above is an average one covering the entire run and the amounts of the various constituents of the bath may var y materially during the operation of the cell. It will be appreciated that this is a commercially pure manganese satisfactory for many industrial uses, but if it is desired to have a purer manganese one will start with a purer material for the cell feed. By having a high enough content of manganese oxide in the bath, the aluminum metal is almost completely consumed and only insignificant amounts may be present in the nal product. The amount of manganese oxide dissolved in the bath will increase with a decrease in the amount of alumina dissolved therein. Since the alumina is not consumed during the process, it therefore is feasible to use only a small amount thereof, and thus provide room for a high concentration of manganese oxide. Due to the fused bath remaining in contact with solid manganese oxide, the bath contains a high concentration of the oxide in solution approaching saturation.

lAs another example, a satisfactory bath and lining composition may comprise 20% MnO, 10% A1203, 40% Fand 30% Na. Also, the manganese oxide content in the wall may equal that of the bath, so that no oxide will dissolve therefrom. In that case, all of the oxide will be` fed into'the top of the furnace. This may be done successfully without arcing at the anode by holding the voltage at 6 to 8 volts and not increasing it to supply further heat to the cell. It is that arcing which causes the formation of carbides. If the voltage is not raised to cause arcing, I may coat the anode with manganese oxide to minimizel the carbon consumption where the exposed part of the anode is in contact with the air above. This anode coating may comprise an excess of powdered MnO periodically shoved down through the bath crust adjacent the anode.

The preferred bath compositions ordinarily result in a cell operating temperature between 920 and 950 C., and even with the excthermic reactions taking place, this is usually so low that the product is found as a mass of separate pellets or as a sponge comprising about 50 to 75% 0f metal intermixed with solidified bath material, the proportions depending upon the temperature as well as the rate at whichA the cathode and cell are lowered.

The manganese metal thus produced, whether in the form of an ingot, pellets or sponge, may be purified by melting in a separate vessel the mass of metal and chilled material clinging to it. At this temperature (manganese melts at 1260 C.) any traces of aluminum or magnesium present areoxidized by the excess of manganese oxide present, and this uid flux is readily separated from the metal. This heating operation may be prolonged. to insure the conversion by the 2,898,589 fluoride present of any trace of silica present to silicon fluoride and its volatilization. In this way, I may obtain a very pure product. Hence, the formation of a sponge or pellets of metal intermixed with the manganese metal has an advantage, and it is not necessary to run the cell at such a high temperature as to make a solid ingot of manganese.

It will be appreciated, rin view `of the above statements, that it is desirable not to use carbon for the cathode or for the cell lining which contacts with the manganese metal on the cathode, so as to minimize any tendency for the formaftion of carbides. This process does not depend in any way upon carbon reduction of themanganese oxide but solely upon electrolysis of al fluoride bathk containing the oxides of manganese and the desired reagent metal and the chemical reaction of that aluminum or magnesium reagent metal with the oxide or its resulting co'mpounds, such as MnFz or- Na2MnF4. Also, it is found that manganese metal is not deposited-to a material extent when its uoride is electrolyzed,

and that the presence of the aluminum or magnesium is necessary for the formation of the metal. Since the ionsof the alkaline metals sodium, calcium, or other alkali metal or alkaline earth metal cannot deposit as a metal at the cathode, these remain in solution and the electrolytic action results only in the deposition of aluminum or magnesium and manganese with an evolution of oxygen at the anode. Hence, the

any particular theory of `chemical or electrof therein a substantial amount of an oxide of a V reagent metal selected from th'e group consisting of aluminumand magnesium, holding the bath fused at a temperature below the melting point of manganese, progressively dissolving manganese oxide in the bath and maintaining a high concentration thereof, progressively separatingthe `anode and the associated fused bath zone from said cathode and the' cell bottom and gradually forming fan ingot of solidified bath stock containing manganese metal which is substantially free from thereagent metal. l

y 2. The method of making manganese compriss ing the steps of electrolyzing between an anode and a metal cathode located therebeneath in a fluoride bath serves merely as an electrolyte as well as a solvent for the reagent materials. There may be some slight .chemical decomposition of the bath at the operating temperatures employed, resulting in a slight volatilization and loss of uorine. This loss may be made up by the addition of a suitable metallic fluoride. such as' cryolite, or the fluoride ofaluminum, manganese, or sodium or other reagent. e

This process applies also to the production of ferro-manganese. In that case, an ore contain` ing both manganese oxide and iron oxide may be introduced into the alkaline metal fluoride bath containing' the aluminum or magnesium oxide dissolved therein. Iron will be collected at the cathodegalong with the manganese, and the relative amounts of the two elements'may be regulated by suitably varying their proportions in the fluid bath. 'Ihe production of ferro-manganese is, therefore, to be considered as falling within the scopeof the claims, since the process of making manganese metal issubstantially the same whether orv not an iron compound is present and metallic iron is formed simultaneously and alloyed with the manganese,

It will also be appreciated that various cell I num may be adopted. Hence, the above description is to be interpreted as illustrative' of the invention and its principles and not as limitations on the claims which are to be interpreted as covering the full equivalents of materials andv process steps. Also, while I have endeavored to explain this invention in the light of present ace cepted theories, 1t is to be understood that the claims are not to be interpreted as dependenton another.

cell a fused bath composed chiefly of a substance containing uorine combined with aluminum and a `metal selected from the group consisting of potassium, sodium and calcium, said bath containing about 2 to 5% by weight of alumina dissolved therein, maintaining the bathfus'ed at a temperature below the melting point of manganese, contacting the bath with solid manganese oxide and progressively dissolving-said oxide and maintaining a high concentration approaching saturation to insure that the ultimate metal product be substantially wholly manganese, progressively separating the anode and the fused bath from the cathode and cell bottom therebeneath.

and gradually building up an ingot of solidified bath-stock and metal composed of manganese substantially free from aluminum.

3. The method of making manganese comprising the steps of electrolyzing between an anode yand a metal cathode therebeneath in an open c`ell a fused bath composed chiey of cryolite whichcontains at least `2% `of alumina dissolved therein, holding the bath fused at a temperature below the melting point of manganese, continu-k ously contacting the bath with solid manganese oxide and progressively feeding said oxide into the open top of the cell and maintaining a high concentration of dissolved manganese oxide approaching saturation to insure that the ultimate metal product be substantially wholly manganese, ,progressively moving 'the anode and the fused bath away from said cathode and the cell bottom and gradually building up an ingot of solidified bathstock containing metal composed substantially wholly of manganese, and supplying bath stock to maintain a predetermined volurne of fused electrolyte in the cell.

4. The method according to claim 1 in which the manganese metal product and its impurities are heated with the associated bath stock containing said fluorides and manganese oxide to a temperature above the melting point of the manganese to complete the reactions and purify the metal and the manganese from the residue.

` THOMAS A. MITCHELL.

is thereafter separated 

